Hi

I am taking it that by IRS you mean Independent Rear Suspension. The answer to your question is a yes and no. For starters if you REALLY want to fit back end off a Sierra into a Capri you could do it (if you're into good welding n stuff) however I do not recommend it, because it won't handle as good as you might expect. You might end up making your Capri handle like a pig.

If you're looking to improve the handling of your nice 300 BHP baby there is other routes you can try depending on your budget.

First Step Starting from the most economical one, a good set of single leaf springs off a 3 litre MK3 with polyurethane bushes and A frame arrangement will drastically improve your handling. If you got some more cash there are some coilover kits which can be mounted on to your existing rear dampers but this does not mean that you will eliminate the leaves. I know of some people who tried it. The only effect they got is harder suspension at the back thats all. This would help relocate the axle under heavy cornering and would eliminate some other axle movement.

Second step up (if you got some more cash to spare that is). You can put in some nice adjustable shockers from GAZ (which are currently installed on mine) and they are good value for money.

Third step, if you're into drag races and stuff like that (so racing on a straight track). I DO NOT RECOMMEND that you remove the live axle because it is far better on the straights than the Sierra item. However if you want to keep the Capri's axle from twisting upwards due to the high torque, you can put in some lifters OR an Air suspension system which locates between the live axle and the chassis. Lifters can keep the axle fixed from twisting, but will also harden the Capri's ride completely. The advantage of air suspsnsions is that you can fill them up with air(through some motorized air compressor) stiffen the back end as much as you want, race a couple of times and then release the air again thus getting all the soft ride back again.

Fourth step if you REALLY like your Capri and you have some extra cash to spare I recommend that you talk to Ric Wood (these may be found at http://www.ricwood.com/capri.htm). These guys specialise in Capri performance as well as handling and they did some quite nice jobs out there. They will typically turn your Capri into a proper race car and optimise it for maximum handling. It won't be cheap but its good value for money.

I hope that thie post was of help to you. Feel free to take a look at my build

Cheers